Common Problems with Facebook Offline Conversions on Zapier
If you’ve updated to a Meta work account, you’ll need to reconnect to any previously connected Meta accounts in your Zapier account. Learn more about how to reconnect your account and prevent disruption to your Zaps.
I'm getting a 400 error when sending an offline event
Do you have the Event Name selected as Purchase? If so, you will need to have "Value" and "Currency" filled out with a value as well.